1. Field of the Invention
This invention concerns a process for the industrial production of urea by reacting ammonia (NH.sub.3) and carbon dioxide (CO.sub.2) in a synthesis section consisting of many differentiated yield reaction spaces, in one of these spaces a majority part of the total conversion occurs by feeding it with highly pure reagents, and in the other of these spaces the remaining minority part of the conversion occurs by feeding it with less pure reagents that have been substantially recirculated by a recovery section.
This invention comprises also the plants for the installation of said process.
2. Description of the Related Art
A method for the synthesis of urea of the type set forth in the introduction is described in the Swiss patent application No. 03216/90-1 deposited on Oct. 3rd, 1990, by the Applicant.
The plant (a "once through type") for the carrying out of this process comprises: a first high-yield reactor fed with CO.sub.2 and fresh NH.sub.3 from the outside and with very pure recovery NH.sub.3 ; a second reactor, parallel with the first, with a less high yield than the first and substantially fed with reagents from the recovery mixture; and a system or recovery section for the recovery of reaction mixtures obtained from said first and second reactors.
In another Swiss patent application, No. 00264/91-4, filed Jan. 29, 1991, the Applicant has described an embodiment of the process according to said first patent application, which lends to a particularly efficient and advantageous result, because of the small investment and minimum consumption of energy it requires, characteristically the majority synthesis reaction stage (A) with high-yield, between highly pure reagents, operating at a higher pressure (Pmax), for instance, above 300 abs and preferably around 400 bar abs, is followed by a flash stage F1 operating at pressures lower than approximately 200 bar abs, the gaseous effluent GF1 of the above mentioned flash stage F1 being fed to a minority synthesis reaction stage B with less pure reagents operating at a pressure lower than 200 bar abs, while the liquid effluent EL1 of the above mentioned flash stage, together with the effluent EB from minority reaction stage B operating in parallel to the majority reaction stage A, are fed to a recovery section RE consisting of two decomposition stages D1 and D2 operating in series: the first D1 being lower than 100 bar abs preferably at 50 bar abs; the second D2 working at a pressure lower than 50 bar abs preferably at 20 bar abs. Each decomposition stage consists of a decomposer D1 and D2 respectively (heat exchangers for the distillation of reagents not transformed into urea), whose gas effluents, consisting of NH.sub.3 +CO.sub.2 +H.sub.2 O, feed a condensation system with direct heat recovery from the process, where the partial condensation of said effluents is carried out and then completed in a fractionating column with a head condenser.